This invention relates to a digital envelope generator for a polyphonic music synthesizer of the voltage controlled type.
Exponentially decaying waveforms, which are employed to simulate plucked or struck strings; or to simulate percussive effects; are commonly produced by analog circuits using resistive-capacitive circuits. The number of components required was fairly reasonable for the monophonic type synthesizers produced in the past, but has become burdensome in the more recent polyphonic synthesizers. The use of digital techniques to reduce the need for numerous discrete components form the basis for the present invention.
One prior proposal using digital techniques is disclosed in U.S. Pat. No. 3,819,844 by Sigeki Isii. In this patent different valued resistors are sequentially switched into the shunt leg of a voltage divider connected in the tone signal path of each note to be keyed. A second voltage divider is connected in cascade with the first and is controlled by a timer to scale the signal envelope in accordance with the velocity of the playing key during its depression. One serious disadvantage of this arrangement is that an individual switch and resistor must be provided for each attenuation level desired, thereby limiting the range unless very large steps are used. Another difficulty is the necessity of providing a dedicated keyer for every note of the keyboard.
Another digital approach is that disclosed in U.S. Pat. No. 3,515,792 by Ralph Deutsch. This digital organ patent discloses an envelope generator which functions by shifting the digital words representing the tone signal samples to the right by a programmed number of positions, thereby attenuating the signal by 6 db for each shift operation. These steps are too large for use with musical tones having substantial decay times, such as piano tones.
Ralph Deutsch has proposed another type of envelope generator in U.S. Pat. No. 3,952,623, wherein a read-only memory storing a set of attack/decay scale factors in digital form is accessed at successive locations to obtain progressively increasing/decreasing scale factors which are used to scale the amplitude of the tone waveshapes by means of a digital multiplier. This approach requires a memory location for each step of the envelope, which is an inefficient use of memory when many steps are required.
Implementing any of these prior approaches in LSI (large scale integration) form requires the use of custom designs, hence involves a large development expense. Furthermore, both of the Deutsch patents disclose digital organs wherein the envelope is a series of digital words that are multiplied by the digital representations of a tone waveform, summed with similar products for other notes, and finally output through a D/A converter to an audio system. The numerous multiply operations involved in this approach requires the use of a much more sophisticated and higher speed computer than that required by the present invention.